1. Field of the Invention
The present invention relates to an improved sample gas extracting apparatus for use in the analysis of ingredients contained in an exhaust gas discharged from, for example, an internal combustion engine or the like.
2. Prior Art
A representative conventional sample gas extracting apparatus is shown in FIG. 1. This apparatus consists of a sample gas line 21, which carries a mixture of an exhaust gas and a diluent gas, e.g. air, in the direction of the arrow and provided with a Critical Flow Venturi (CFV)20; a suction apparatus 22 for maintaining the pressure difference between the upstream and downstream ends, relative to the directions of the flow of the gas, of said CFV 20 at a value higher than the critical differential pressure in said CFV 20, and a sample gas extracting line 23 for extracting a quantity of the sample gas supplied to the upstream end of said CFV 20 from said sample gas line 21. At this time, since the critical gas flow rate Q.sub.1 of said CFV 20 and the quantity Q of the sample gas in said sample gas line 21 are known, the sample gas can be extracted into said sample gas extracting line 23 at a ratio corresponding to the ratio of Q/Q.sub.1 1.
Said CFV 20 is replacable by previously prepared CFV's 20', 20" . . . having different critical gas flow rates Q.sub.1.
However, this conventional apparatus has the follow defects:
Although said suction apparatus 22, which has a sufficient suction force for holding said CFV 20 at the critical differential pressure when said sample gas line 21 has a CFV 20 with the largest critical gas flow rate Q.sub.1 therein, when such CFV 20 is replaced by one having a smaller critical gas flow rate Q.sub.1, a surging phenomenon is produced in said suction apparatus 22 at a critical gas flow rate Q.sub.1 at which about 1/3 the suction apparatus is performing at the suction capacity. This makes necessary the addition of a surge preventing apparatus 27 consisting of a throttle valve 24, a stop valve 25 and an air filter 26 or the like through which supplemental air is supplied into line 21 downstream of CFV 20. Said surge preventing apparatus 27 must be larger than the suction capacity of said suction apparatus 22. The surge preventing apparatus 27 thus makes necessary a larger size installation space for the overall apparatus.
In addition, there has been a problem during operation since said CFV 20 must be replaced every time the extracting ratio of sample gas, that is to say the flow rate in said sample gas line 21 is changed.